Method for in-situ manufacture of a lightweight fly ash based aggregate

ABSTRACT

A method of making a rapid setting lightweight homogeneous foamed fly ash based cementitious aggregate composition with improved compressive strength for products such as panels is disclosed. The method mixes fly ash, alkali metal salt of citric acid, foaming agent for entraining air, optional foam stabilizing agent, a calcium sulfate such as stucco or gypsum, and water. Compositions are also disclosed which include mixtures of fly ash, particularly Class C fly ash alone or in mixtures with Class F fly ash, alkali metal salts of citric acid, foaming agents, a calcium sulfate such as calcium sulfate dihydrate or hemihydrate and an optional portland cement.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/428,819, filed Dec. 30, 2010 incorporated herein by reference in itsentirety and is related to:

U.S. Provisional Application No. 61/428,839 entitled Lightweight FoamedFly Ash Binders, filed Dec. 30, 2010; incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

This invention relates generally to fast setting cementitiouscompositions that can be used for a variety of applications in whichrapid hardening and attainment of early strength is desirable. Inparticular, the invention relates to a method of making a homogenouslightweight fly ash based cementitious aggregate composition which hasproperties similar to expanded clay aggregates and lightweight fillersthat can be used to make boards with excellent moisture durability foruse in wet and dry locations in buildings. These aggregates plus apre-formed foam are added to a fast setting cementitious mixture so thatprecast board products can be handled soon after the cementitiousmixture is poured into a stationary or moving form or over acontinuously moving belt. Ideally, this setting of the cement mixturemay be achieved as soon as about 20 minutes, preferably as soon as 10 to13 minutes, more preferably as soon as 4 to 6 minutes, after mixing thecement mixture with a suitable amount of water.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,869,474 to Perez-Pena et al, incorporated herein byreference, discusses extremely fast setting of cementitious compositionsfor producing cement-based products such as cement boards achieved byadding an alkanolamine to hydraulic cement such as portland cement, andforming a slurry with water under conditions that provide an initialslurry temperature of at least 90° F. (32° C.). Additional reactivematerials may be included such as high alumina cement, calcium sulfateand a pozzolanic material such as fly ash. The extremely rapid setpermits rapid production of cementitious products. Triethanolamineadditions have been found to be a very powerful accelerator capable ofproducing formulations with relatively short final setting times withincreased levels of fly ash and gypsum and without the need of calciumaluminate cements. However, formulations with triethanolamine containmainly hydraulic cements such as portland cement and gypsum as thereactive powder, which limits the availability of aluminate phasescompared to the aluminate phases produced by the activation of fly ashmaterials in the present invention.

Pending U.S. patent application Ser. No. 11/758,947 filed Jun. 6, 2007of Perez-Pena et al, incorporated herein by reference, discussesextremely fast setting of cementitious compositions with early-agecompressive strength for producing cement-based products such as cementboards achieved by adding an alkanolamine and a phosphate to a hydrauliccement such as portland cement, and forming a slurry with water underconditions that provide an initial slurry temperature of at least 90° F.(32° C.). Additional reactive materials may be included such as highalumina cement, calcium sulfate and a pozzolanic material such as flyash. Again, all of the compositions contained a significant amount ofhydraulic cement and gypsum.

Pending U.S. patent application Ser. No. 12/237,634 filed Sep. 25, 2008of Perez-Pena discloses formulations using fly ash and alkali metalsalts of citric acid such as sodium citrate to form concrete mixes withfast setting time and relatively high early age compressive strength.One of the challenges encountered with the activated fly ash bindersdescribed in this application, is an apparent pessimum interactionbetween these binders and the traditional foaming systems used toentrain air and thereby make lightweight boards. The fly ash basedbinders which have been made with traditional foams in accordance withthis disclosed method have suffered foam collapsing and/or drasticstrength reduction.

U.S. Pat. No. 4,488,909 to Galer et al, incorporated herein byreference, discusses cementitious compositions capable of rapid setting.The compositions permit high speed production of carbon dioxideresistant products by forming essentially all of the potentialettringite within about 20 minutes after the composition is mixed withwater. The essential components of the cementitious composition areportland cement, high alumina cement, calcium sulfate and lime.Pozzolans such as fly ash, montmorillonite clay, diatomaceous earth andpumicite may be added up to about 25%. The cement composition includesabout 14 to 21 wt % high alumina cement, which in combination with theother components makes possible the early formation of ettringite andother calcium aluminate hydrates responsible for early setting of thecementitious mixture. In their invention, Galer et al providedaluminates using high alumina cement (HAC) and sulfate ions using gypsumto form ettringite and achieve rapid setting of their cementitiousmixture.

Ettringite is a calcium aluminum sulfate compound having the formulaCa₆Al₂(SO₄)₃.32H₂O or alternatively 3 CaO.Al₂O₃.3CaSO₄.32H₂O. Ettringiteforms as long needle-like crystals and provides rapid early strength tocement boards, so that they can be handled soon after being poured intoa mold or over a continuous casting and forming belt.

In general, Galer et al's rapid setting formulation suffers from severallimitations. These limitations, as highlighted below, are even more of aconcern for the production of low cost concrete products such aslightweight aggregates because it uses relatively expensive high aluminacements to provide aluminate phases.

U.S. Pat. No. 5,536,310 to Brook et al disclose a cementitiouscomposition containing 10-30 parts by weight (pbw) of a hydraulic cementsuch as portland cement, 50-80 pbw fly ash, and 0.5-8.0 pbw expressed asa free acid of a carboxylic acid such as citric acid or alkali metalsalts thereof, e.g., tripotassium citrate or trisodium citrate, withother conventional additives, including retarder additives such as boricacid or borax, which are used to accelerate the reaction and settingtime of the composition to overcome the disclosed disadvantageous ofusing a high fly ash content in cement compositions.

U.S. Pat. No. 5,536,458 to Brook et al disclose a cementitiouscomposition containing a hydraulic cement such as portland cement, 70-80parts by weight fly ash, and 0.5-8.0 pbw of a free carboxylic acid suchas citric acid or an alkali metal salts thereof e.g. potassium citrateor sodium citrate, with other conventional additives including retarderadditives such as boric acid or borax, which are used to accelerate thereaction and setting time of the composition to overcome the knowndisadvantageous of using a high fly ash content in cement compositions.

U.S. Pat. No. 4,494,990 to Harris discloses a cementitious mixture ofportland cement e.g. 25-60 pbw, fly ash e.g. 3-50 pbw and less than 1pbw of sodium citrate.

U.S. Pat. No. 6,827,776 to Boggs et al disclose a hydraulic cementcomposition comprising portland cement, fly ash, which has a settingtime controlled by pH of an activator slurry of an acid, preferablycitric acid, and a base which can be an alkali or alkaline earth metalhydroxide or salt of the acid component.

U.S. Pat. No. 5,490,889 to Kirkpatrick et al disclose a blendedhydraulic cement consisting of water, fly ash (50.33-83.63 pbw),portland cement, ground silica, boric acid, borax, citric acid(0.04-2.85 pbw) and an alkali metal activator, e.g. lithium hydroxide(LiOH) or potassium hydroxide.

U.S. Pat. No. 5,997,632 to Styron discloses a hydraulic cementcomposition containing 88-98 wt. % fly ash, 1-10 wt. % portland cementand from about 0.1-4.0 wt. % citric acid. Lime to achieve a desirableminimum lime content of 21% is provided by the subbituminuous fly ash orthe sub-bituminous fly ash in combination with a beneficiating agent. Inaddition to citric acid Styron uses an alkali source such as potassiumor sodium hydroxide.

The final setting times of the cementitious mixtures of prior artproducts are typically greater than 9 minutes and can extend to 2-3hours for standard concrete products. The final setting time is normallydefined as the time in which the cementitious mixtures set to the extentthat the concrete products made thereof can be handled and stacked,although chemical reactions may continue for extended periods.

There is a need to find a method to reduce the weight of fly ash basedbinder mixes so these formulations can be used to manufacture oflightweight cementitious concrete products for applications such asbacker board and other wall or ceiling applications with improvedstrength. The present method has developed formulations with enhancedcompressive strength at reduced weight and with reduced cost.

When lightweight concrete is made, a lightweight aggregate like expandedclay or perlite is generally used rather than sandy gravel or crushedstone. The expanded clay/perlie particles (nodules) are produced by asophisticated pyrogenic process whereby geochemically specific clay orperlite is expanded in a rotary kiln at high temperatures. The expandedclay or perlite particles are extremely lightweight granular aggregatewith a hard vitrified outer shell and an air filled honeycombed innercore. The expansion of the combined water in the crude clay or perlite(rock), results from the rapid heating of the crude rock to temperaturesabove 1600° F. (871° C.), when the rock cracks and combined watervaporizes like popped corn.

The present invention provides a method of making fast settingcementitious slurry used to manufacture lightweight aggregate particles(nodules) at a relatively low temperature. The lightweight aggregateparticles (nodules) can form in situ in a cementitious mixture. Settingthe mixture results in a solid product comprising particles (nodules)formed in situ in the matrix of cementitious material. This product canbe used as is or crushed to form loose lightweight aggregate particles.

The particles are extremely lightweight with a hard outer cementitiousshell with an air filled crystalline inner core. The lightweightparticles provide a low energy, low cost lightweight filler which isideal for manufacturing lightweight cement panels, block or otherlightweight concrete articles.

The invention also provides a lightweight cementitious composition withreduced weight and enhanced early and final compressive strength. Thecementitious composition is formed from a foamed binder solutioncontaining sodium citrates, sodium silicates, foaming agents, foamstabilizer and a reactive powder comprising Class C fly ash and calciumsulfate.

The present invention includes a method of providing a lightweightcementitious mixture having rapid set, improved compressive strength andwater durability comprising: mixing at ambient or above ambienttemperatures, water, cementitious reactive powder, a set acceleratingamount of alkali metal salt of citric acid, and in situ forming areactive powder lightweight aggregate, wherein the weight ratio of waterto reactive powder solids is about 0.17 to 0.35:1.0, or about 0.17 to0.27:1.0, and more preferably about 0.20 to 0.25:1.0. The cementitiousreactive powder comprises fly ash. and a calcium sulfate selected fromthe group consisting of calcium sulfate hemihydrate, calcium sulfatedihydrate, and mixtures thereof, and preferably no hydraulic cement,e.g., no portland cement. Typically essentially 100 wt. of the fly ashis in the form of class C fly ash and blends of class C and class F flyash. For purposes of the present specification cements are characterizedas hydraulic or non-hydraulic. Hydraulic cements (e.g., Portland cement)harden because of hydration, chemical reactions that occur independentlyof the mixture's water content; they can harden even underwater or whenconstantly exposed to wet weather. The chemical reaction that resultswhen the anhydrous cement powder is mixed with water produces hydratesthat are not water-soluble. Non-hydraulic cements (e.g., lime, stucco,gypsum/landplaster and gypsum plaster) must be kept dry to retain theirstrength. Typically the mixture has a wet (water included) density ofabout 40 to 65 pounds per cubic foot, for example, 46 to 51 pounds percubic foot.

The method generally further includes setting the mixture to form asolid product containing the in situ formed aggregate particles. Thesolid product can be used as is or can be broken up, for example bycrushing, to form loose aggregate particles of the present invention.

This cementitious reactive powder includes at least fly ash and stuccoor gypsum/landplaster (stucco is calcium sulfate hemihydrate, gypsum iscalcium sulfate dihydrate) and may also contain ordinary portland cement(OPC), calcium aluminate cement (CAC) (also commonly referred to asaluminous cement or high alumina cement), and a non-fly ash mineraladditive. However, typically there is no added ordinary portland cement(OPC) or calcium aluminate cement (CAC).

Class C fly ash generally contains lime. Thus, the reactive powder blendof the cementitious composition is typically free of externally addedlime.

The preferred initial slurry temperatures are from room temperature toabout 100° F.-115° F. (24° C. to about 38°-46° C.).

The final setting time (i.e., the time after which cementitious boardscan be handled) of the cementitious composition as measured according tothe Gilmore needle should be at most 20 minutes, preferably 10 to 13minutes or less, more preferably about 4 to 6 minutes after being mixedwith a suitable amount of water. A shorter setting time and higher earlyage compressive strength helps to increase the production output andlower the product manufacturing cost.

The very fast setting cementitious compositions of this invention can beused for a variety of applications in which rapid hardening andattainment of early strength is desirable. Using the alkali metal saltof citric acid, such as potassium citrate and/or sodium citrate, toaccelerate setting of the cementitious composition, when the slurry isformed at elevated temperatures, makes possible increased rate ofproduction of cementitious products such as cement boards.

The dosage of alkali metal citrate in the slurry is in the range ofabout 1.5 to 6 wt. %, preferably about 1.5 to 4.0 wt. %, more preferablyabout 2 to 3.5 wt. %, and most preferably about 3.5 wt. % based on thecementitious reactive components of the invention. Sodium citrates arepreferred, although potassium citrate or a blend of sodium and potassiumcitrate can be used. As mentioned above, these weight percents are basedon 100 parts by weight of the reactive components (cementitious reactivepowder). Thus for example, for 100 pounds of cementitious reactivepowder, there may be about 1.5 to 4.0 total pounds of sodium citrates.

A typical cementitious reactive powder of this invention comprises 75 to100 wt % fly ash and 0 wt. % hydraulic cement. Typically at least halfof the fly ash is Type C fly ash.

A cementitious reactive powder of this invention can also comprise classF fly ash up to 46 wt % when mixed with a sufficient amount of class Cfly ash, sodium citrate and optional Portland cement of 0 to 20 wt % tomake up for the lower compressive strength of the class F fly ashcompared to the higher alumina and lime content of the preferred class Cfly ash. When higher amounts of class F fly ash are used, i.e. up to 60wt % of the reactive powder, it has been found the lower compressivestrength of the class F fly ash can not be sufficiently increased byonly adding class C fly ash to the class F fly ash. Thus, although notpreferred, class F fly ash can be used in amounts up to about 60 wt % ifPortland Cement, e.g. Type III Portland cement, is used with the class Ffly ash to increase the compressive strength by addition of alumina andlime beyond the levels typical found in Class C fly ash. Thus, forexample, 46 to 60 wt % class F fly ash could be used with 10 to 32 wt %class C fly ash without compromising compressive strength if about 10 to29 wt % of additional Type III Portland cement is also used with the flyash mixture and 2-4 wt. % sodium citrate.

There is a synergistic interaction between the alkali metal citrate andthe fly ash. In particular, adding alkali metal citrates to fly ashimproves mix fluidity unlike other accelerators such as aluminumsulfate, which can lead to premature stiffening of concrete mixtures.

Other additives, e.g., inert aggregate, may also be present, which arenot considered cementitious reactive powder, but are part of the overallcementitious composition. Such other additives include one or more ofsand, aggregate, lightweight fillers, water reducing agents such assuperplasticizers, set accelerating agents, set retarding agents,air-entraining agents, foaming agents, shrinkage control agents, slurryviscosity modifying agents (thickeners), coloring agents and internalcuring agents, may be included as desired depending upon the processability and application of the cementitious composition of theinvention.

The lightweight cementitious compositions of the present invention canbe used to make precast concrete building products such as cementitiousboards with excellent moisture durability for use in wet and drylocations in buildings. The precast concrete products such as cementboards are made under conditions which provide a rapid setting of thecementitious mixture so that the boards can be handled soon after thecementitious mixture is poured into a stationary or moving form or overa continuously moving belt.

The lightweight cementitious compositions can be used in any concreteproduct application including concrete panels, flooring, overlays,finishes, capping, as well as patching mixes for concrete roads. Theconcrete products made with the lightweight compositions of thisinvention have particular advantages for use which require waterdurability compared to compositions which contain gypsum andapplications which require higher compressive strength than cementcontaining compositions which have a higher carbon foot print.

Fly ash material is mostly aluminosilicates. Thus, it is theorized thelightweight aggregate of the invention may be similar to that of themost expensive perlite or expanded clay aggregate.

All percentages, ratios and proportions herein are by weight, unlessotherwise specified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of a 10× magnification of the homogeneousparticles of a mixture (#3) of 65% class C fly ash and 35% stucco.

FIG. 2 is a photograph of a 20× magnification of the homogeneousparticles of mixture (#3) of 65% class C fly ash and 35% stucco.

FIG. 3 is a photograph of a 10× magnification of the wide distributionof particles of around 0.3 up to 2.0 mm in size in a mixture (#5) of 70%class C fly ash and 30% landplaster.

FIG. 4 is a photograph of a 20× magnification of the mixture (#5) of 70%class C fly ash and 30% landplaster, wherein the particles of about 1.0mm are not as spherical in shape as the larger particles.

FIG. 5 is a photograph of a 10× magnification of a mixture (#6) of 65%class C fly ash and 35% landplaster showing the particles have adistribution of particle sizes of about 1.0 up to 2.0 mm.

FIG. 6 is a photograph of a 20× magnification of mixture (#6) of 65%class C fly ash and 35% landplaster showing spherical particles of about1 mm in size.

FIG. 7 is a SEM (scanning electron microscope) photograph which showsmatrix fly ash particles surrounded by mostly crystalline phase withrelatively less of the glassy phase for a sample of 70% Class C Fly ash30% Gypsum, 4.5% sodium citrate W/FA=0.20, R.T. Wet density=50 pcf,Witconate AOS soap, made by mixing soap and sodium citrate solution withfly ash for about 3 minutes.

FIG. 8 a and FIG. 8 b are SEM photographs showing inside aggregateparticle similar microstructure as compared to outside (FIG. 7) butrelatively denser packing, wherein FIG. 8 a is a Secondary electronimage and FIG. 8 b is a Backscatter electron image.

FIG. 9 is a SEM photograph showing gypsum encapsulated inside aggregateparticle (same particle shown in FIG. 8 a, b).

FIG. 10 a and FIG. 10 b are SEM photographs of a matrix composed ofglassy phase with nanoparticles immersed in it for a sample having ananalysis of 100% Class C Fly ash, 0% Gypsum, 4.5% sodium citrate,W/FA=0.20, R.T. Wet density=47 pcf, Witconate AOS soap, made by mixingsoap and sodium citrate solution with fly ash for about 3 minutes.

FIG. 11 a and FIG. 11 b are SEM photographs of the sample shown in FIG.10 a, b at a higher magnification showing the matrix also includescrystal phase surrounded by glassy phase, wherein FIG. 11 a is at amagnification of X=15,000 and FIG. 11 b is at a magnification ofX=30,000.

DETAILED DESCRIPTION OF THE INVENTION

The present invention includes a method of providing a lightweightcementitious mixture having improved compressive strength and waterdurability comprising: mixing water, reactive powder, an alkali metalsalt of citric acid, and in situ forming lightweight aggregate whereinthe weight ratio of water to reactive powder solids is about 0.17 to0.35:1.0, typically about 0.17 to 0.30:1.0, more preferably about 0.2 to0.23:1.0. The reactive powder typically comprises 65 to 100 wt. % (or 75to 100 wt. %) fly ash and 0 to 35 wt. % (or 0 to 25 wt. %) hydrauliccement and/or gypsum, typically a calcium sulfate selected from thegroup consisting of calcium sulfate hemihydrate, calcium sulfatedihydrate, and mixtures thereof. Typically the present invention mixesthe cementitious reactive powder including fly ash with potassiumcitrates and/or sodium citrates and water at an initial slurrytemperature of at least room temperature to 115° F. (24° C. to 41° C.)to yield a rapid set of preferably less than 10 to 13 minutes, morepreferably about 4 to 6 minutes or less. The lightweight aggregateparticles (nodules) form in situ in the cementitious mixture. Settingthe cementitious mixture forms a solid product of the aggregateparticles in a matrix of cementitious material (cementitious binder).The matrix being the portion of the cementitious material of the mixturethat did not form aggregate particles (nodules). If desired this productof particles (nodules) formed in situ in a matrix of cementitiousmaterial can be crushed to form loose lightweight aggregate particles.

The present invention also provides cementitious compositions withenhanced rapid final setting performance and enhanced early compressivestrength.

The typical ingredients of the composition for making the aggregate ofthe present invention are listed in the following TABLE A.

TABLE A More Most Broad Preferred preferred preferred parts by parts byparts by parts by weight weight weight weight dry basis dry basis drybasis dry basis per 100 per 100 per 100 per 100 parts parts parts partsreactive reactive reactive reactive Ingredient powder powder powderpowder Reactive Powder (total) 100 parts 100 parts 100 parts 100 partsFly Ash (class C) 50-95  60-95  65-80 65-75 Class F Fly Ash 0-30 0-20 0-20 0 Portland Cement 0-30 0-20  0-20 0 Calcium sulfate 5-40 5-4020-40 25-35 dihydrate or hemihydrate (gypsum) Calcium aluminate OptionalNone None cement non-fly ash mineral Optional None None additive addedlime Optional None None alkali metal salt of citric 1.5 to 6   1.5 to 41.5 to 4 2.0 to 3.5 acid Foam/air-entraining 2.0 to 6.0 3-5  3-5 4 agent(alpha olefin sulfonate soap) Polyvinyl alcohol optional stabilizerSuperplasticizer optional *added lime not needed if reactive powderingredients already contain sufficient lime.

Generally the weight ratio of water to cementitious reactive powder isabout 0.15 to 0.3:1.0. Inert lightweight aggregates are not part of thecementitious reactive powder.

While not wishing to be limited to a particular theory, it is theorizedthe lightweight aggregate particulates are formed as the calciumaluminate phases are leached out from the dissolving fly ash particles,with high fly ash mineral content of 75 to 100 wt % and no portlandcement or calcium aluminate cement. The leaching out occurs after mixingthe cementitious reactive powder, alkali metal citrate, calcium sulfateaggregate and water to form slurry at temperatures of about 20° C. soformation of calcium sulphoaluminate hydrates and/or hydrates of aluminosilicate takes place as a result of the hydration of this reactivepowder blend with the alkali metal citrate.

Thus, a suitable amount of water is provided to hydrate the cementitiousreactive powder and to rapidly form calcium sulphoaluminate alkalihydrates and other hydrates present in the fly ash. Generally, theamount of water added will be greater than theoretically required forthe hydration of the cementitious reactive powder. This increased watercontent facilitates the workability of the cementitious slurry.Typically, in the slurry the weight ratio of the water to reactivepowder blend is about 0.20 to 0.35:1, more typically about 0.20 to0.30:1, preferably about 0.20 to 0.23:1. The amount of water depends onthe needs of the individual materials present in the cementitiouscomposition.

The calcium sulphoaluminate hydrates and/or other hydrates of aluminosilicate and/or calcium alumino silicate compounds form very rapidly inthe hydration process thus imparting rapid set and rigidity to theaggregate particles The aggregate particles made this way will have arelatively low density because of the relatively large volume occupiedby the needle like microstructure of the calcium sulphoaluminateminerals which are formed within a few minutes after the cementitiouscomposition of the invention is mixed with a suitable amount of water.

Setting of the composition is characterized by initial and final settimes, as measured using Gilmore needles specified in the ASTM C266 testprocedure. The final set time also corresponds to the time when aconcrete product, e.g., a concrete panel, has sufficiently hardened sothat it can be handled or trafficked, in the case of a concrete floor orroad. Relatively higher early age (3 to 5 hours) compressive strengthcan be an advantage for concrete material because it can withstandhigher stresses without deformation. It will be understood by thoseskilled in the art that curing reactions continue for extended periodsafter the final setting time has been reached.

Early age strength of the composition is characterized by measuring thecompressive strength after 3 to 5 hours of curing as specified in theASTM C109. Achieving high early strength allows for ease of handling thestacked panels.

Cementitious Reactive Powder

The cementitious reactive powder contains fly ash and optionally non-flyash mineral additives which are mixed with a calcium sulfateparticularly calcium sulfate dihydrate (stucco), calcium sulfatehemihydrate (gypsum or landplaster) or mixtures thereof. Thecementitious reactive powder typically contains 65 to 100% fly ash,preferably Class C fly ash, and 0 to 35 wt. % of a member selected fromthe group consisting of calcium sulfate hemihydrate, gypsum and mixturesthereof with optional non-fly ash mineral additives. The cementitiousreactive powder preferably contains 60-95 wt % fly ash, preferable classC fly ash, and 5-40 wt % calcium sulfate in the form of calcium sulfatehemihydrate and/or calcium sulfate dihydrate. In a more preferredreactive powder of the invention, the reactive powder comprises 65-75 wt% class C fly ash, 25-35 wt % calcium sulfate hydrates and no hydrauliccement.

Preferably the cementitious reactive powder contains 10 to 40 wt. lime.However, this lime is generally not added lime. Rather it is included inanother ingredient of the cementitious reactive powder, for example, thefly ash.

The principal ingredient of the cementitious reactive powder of thecementitious composition of the invention is a fly ash mineral additive,preferably Type C fly ash. Fly ash is described below in the sectionentitled Fly ash and Non-fly ash Mineral Additives.

In addition to fly ash, the cementitious reactive powder may include 0to 25 wt. % of optional cementitious additives such as portland cement,calcium aluminate cement, calcium sulfate or gypsum (landplaster).However, the lower water content cementitious compositions of theinvention, i.e. cementitious compositions with a water to reactivepowder weight ratio of about 0.17 to 0.35:1.0, with these optionalcementitious additives have a significantly reduced compressive strengthcompared to the same lower water content compositions of the inventionwithout the additional cementitious additives.

For example, in some cementitious reactive powder blends whencompressive strength is not required or when higher water to reactivepowder ratios are to be used e.g. at ratios above about 0.35:1.0,portland cement can be used at about 0 to 25 wt % and fly ash 75 to 100wt %.

Fly Ash and Non-Fly Ash Mineral Additives

The hydraulic cement of traditional reactive powder compositions issubstantially replaced by fly ash having pozzolanic properties,particularly Class C fly ash although blends of Class C and Class F flyash can be used without hydraulic cement provided the amount of class Ffly ash is below 46 wt. % and preferably 30 wt % of the fly ash blend.Other optional non-fly ash mineral additives possessing substantial,little, or no cementing properties can be added. When added, non-fly ashmineral additives having pozzolanic properties are preferred in thecementitious reactive powder of the invention.

ASTM C618-97 defines pozzolanic materials as “siliceous or siliceous andaluminous materials which in themselves possess little or nocementitious value, but will, in finely divided form and in the presenceof moisture, chemically react with calcium hydroxide at ordinarytemperatures to form compounds possessing cementitious properties.”Various natural and man-made materials have been referred to aspozzolanic materials possessing pozzolanic properties. Some examples ofpozzolanic materials include pumice, perlite, diatomaceous earth, silicafume, tuff, trass, rice husk, metakaolin, ground granulated blastfurnace slag, and fly ash.

All of these pozzolanic materials can be used either singly or incombined form as part of the cementitious reactive powder of theinvention.

Fly ash is the preferred pozzolan in the cementitious reactive powderblend of the invention. Fly ashes containing high calcium oxide andcalcium aluminate content (such as Class C fly ashes of ASTM C618standard) are preferred as explained below. Other mineral additives suchas calcium carbonate, vermiculite, clays, and crushed mica may also beincluded as optional mineral additives.

Fly ash is a fine powder byproduct formed from the combustion of coal.Electric power plant utility boilers burning pulverized coal producemost commercially available fly ashes. These fly ashes consist mainly ofglassy spherical particles as well as residues of hematite andmagnetite, char, and some crystalline phases formed during cooling. Thestructure, composition and properties of fly ash particles depend uponthe structure and composition of the coal and the combustion processesby which fly ash is formed. ASTM C618 standard recognizes two majorclasses of fly ashes for use in concrete—Class C and Class F. These twoclasses of fly ashes are generally derived from different kinds of coalsthat are a result of differences in the coal formation processesoccurring over geological time periods. Class F fly ash is normallyproduced from burning anthracite or bituminous coal, whereas Class C flyash is normally produced from lignite or sub-bituminous coal.

The ASTM C618 standard differentiates Class F and Class C fly ashesprimarily according to their pozzolanic properties. Accordingly, in theASTM C618 standard, the major specification difference between the ClassF fly ash and Class C fly ash is the minimum limit of SiO₂+Al₂O₃+Fe₂O₃in the composition. The minimum limit of SiO₂+Al₂O₃+Fe₂O₃ for Class Ffly ash is 70% and for Class C fly ash is 50%. Thus, Class F fly ashesare more pozzolanic than the Class C fly ashes. Although not explicitlyrecognized in the ASTM C618 standard, Class C fly ashes typically havehigh calcium oxide (lime) content.

Class C fly ash usually has cementitious properties in addition topozzolanic properties due to free lime (calcium oxide), whereas Class Fis rarely cementitious when mixed with water alone. Presence of highcalcium oxide content makes Class C fly ashes possess cementitiousproperties leading to the formation of calcium silicate and calciumaluminate hydrates when mixed with water. As will be seen in theexamples below, Class C fly ash has been found to provide superiorresults, particularly in the preferred formulations in which calciumaluminate cement and gypsum are not used.

Typically at least 50 wt. % of the fly ash in the cementitious reactivepowder is Type C fly ash. More typically at least 75-80 wt. % of thecementitious reactive powder is Type C fly ash. Still more preferably atleast 88.5-100 wt. % of the cementitious reactive powder is Type C flyash.

Typical minerals found in fly ash are quartz (SiO₂), mullite(Al₂Si₂O₁₃), gehlenite (Ca₂Al₂SiO₇), haematite (Fe₂O₃), magnetite(Fe₃O₄), among others. In addition, aluminum silicate polymorphsminerals commonly found in rocks such as sillimanite, kyanite andandalusite all three represented by molecular formula of Al₂SiO₅ arealso found in fly ash.

A typical suitable Class C fly ash made from sub-bituminous coal has thefollowing composition listed in TABLE B.

TABLE B Typical Class C fly ash Composition Component Proportion (wt. %)SiO₂ 20-40 Al₂O₂ 10-30 Fe₂O₃  3-10 MgO 0.5-8   SO₃ 1-8 C 0.5-2   H₂O0.33-3   CaO 25-35 K₂O 0.5-4   Na₂O 0.5-6  

The fineness of the fly ash is typically such that less than about 34%is retained on a 325 mesh sieve (U.S. Series) as tested on ASTM TestProcedure C-311 (“Sampling and Testing Procedures for Fly Ash as MineralAdmixture for Portland Cement Concrete”). This fly ash is preferablyrecovered and used dry because of its self-setting nature.

A typical Class F fly ash which be used in the invention has thefollowing composition listed in TABLE C.

TABLE C Typical Class F fly ash composition. Component Proportion (wt.%) SiO₂ 50-65 Al₂O₂ 10-30 Fe₂O₃  3-10 MgO 0.5-3   SO₃ 0.3-8   C 0.25-3  H₂O 0.33-3   CaO  0-10 K₂O 0.5-4   Na₂O 1-6

The composition of a typical Portland Cement Type III which can be usedin the present invention is shown in TABLE D

TABLE D Typical Portland Cement III Component Proportion (wt.) CaO 61.04Al₂O₂ 4.79 Fe₂O₃ 2.70 SiO₂ 19.62 MgO 2.62 SO₃ 4.80 K₂O 1.06 Na₂O 0.37P₂O₅ 0.16 TiO₂ 0.27 TOC (total organic carbon) LOI (950 C.) 1.75 Total99.35 Estimated Cement Phases (Bogue) C₃S (tricalcium silicate) 49.73C₂S (dicalcium silicate) 18.73 C₃A (tricalcium aluminate) 8.11 C₄AF(tetracalcium alumino-ferrite) 8.21 Alkali 1.06

Fly ash makes up substantially all of the cementitious material of thereactive powder of the invention. The addition of other commoncementitious additives are not needed with class C fly ash and have beenfound to reduce the ultimate compressive strength of the lightweightaggregate compositions of the invention.

In the case when Class F fly ash, which has substantially less aluminaand lime content than class C fly ash, is used in place of a substantialamount or all of the Class C fly ash, it has been found the addition ofType III Portland cement is required to increase the compressivestrength of the Class F fly ash binder to the levels obtained with theClass C fly ash based composition, which has substantially more aluminaand lime content. In particular when up to 60 wt % Class F fly ash isused in the binder system, the addition of up to 30 wt % type IIIPortland cement increases the compressive strength of the binder morethan three and a half times more than the addition of only class C flyash to the Class F fly ash. Thus when class F fly ash is used in thepresent binder, the preferred mixture is about 46 to 60 wt % class F flyash, 10 to 29 wt % Type III Portland Cement and 10 to 32 wt % Class Cfly ash and 2 to 4 wt % sodium citrate with water. The ratio of water tofly ash and portland cement should be maintained below 0.37 andpreferably below 0.33.

In the present invention, the need for the use of hydraulic cement, likeType III Portland cement can be avoided, and relatively fast early agestrength development can be obtained using substantial all Class C flyash instead of mixtures of Class F fly ash containing Type III portlandcement. Other recognized types of cements which are not needed in thepreferred Class C fly ash based composition of the invention includeType I portland cement or other hydraulic cements including Type IIportland cement, white cement, slag cements such as blast-furnace slagcement, and pozzolan blended cements, expansive cements, calciumsulfo-aluminate cements, and oil-well cements.

Calcium Aluminate Cement

Calcium aluminate cement (CAC) is a type of hydraulic cement that mayform a component of the reactive powder blend of some embodiments of theinvention when higher compressive strength is not required with lowwater content slurries containing substantial amounts of fly ash.

Calcium aluminate cement (CAC) is also commonly referred to as aluminouscement or high alumina cement. Calcium aluminate cements have a highalumina content, about 36-42 wt % is typical. Higher purity calciumaluminate cements are also commercially available in which the aluminacontent can range as high as 80 wt %. These higher purity calciumaluminate cements tend to be very expensive relative to other cements.The calcium aluminate cements used in the compositions of someembodiments of the invention are finely ground to facilitate entry ofthe aluminates into the aqueous phase so that rapid formation ofettringite and other calcium aluminate hydrates can take place. Thesurface area of the calcium aluminate cement that may be used in someembodiments of the composition of the invention will be greater than3,000 cm²/gram and typically about 4,000 to 6,000 cm²/gram as measuredby the Blaine surface area method (ASTM C 204).

Several manufacturing methods have emerged to produce calcium aluminatecement worldwide. Typically, the main raw materials used in themanufacturing of calcium aluminate cement are bauxite and limestone. Onemanufacturing method that has been used in the US for producing calciumaluminate cement is described as follows. The bauxite ore is firstcrushed and dried, then ground along with limestone. The dry powdercomprising of bauxite and limestone is then fed into a rotary kiln. Apulverized low-ash coal is used as fuel in the kiln. Reaction betweenbauxite and limestone takes place in the kiln and the molten productcollects in the lower end of the kiln and pours into a trough set at thebottom. The molten clinker is quenched with water to form granulates ofthe clinker, which is then conveyed to a stock-pile. This granulate isthen ground to the desired fineness to produce the final cement.

Several calcium aluminate compounds are formed during the manufacturingprocess of calcium aluminate cement. The predominant compound formed ismonocalcium aluminate (CaO.Al₂O₃, also referred to as CA). The othercalcium aluminate and calcium silicate compounds that are formed include12CaO.7Al₂O₃ also referred to as C₁₂A₇, CaO.2Al₂O₃ also referred as CA₂,dicalcium silicate (2CaO.SiO₂, called C₂S), dicalcium alumina silicate(2CaO.Al₂O₃.SiO₂, called C₂AS). Several other compounds containingrelatively high proportion of iron oxides are also formed. These includecalcium ferrites such as CaO.Fe₂O₃ or CF and 2CaO.Fe₂O₃ or C₂F, andcalcium alumino-ferrites such as tetracalcium aluminoferrite(4CaO.Al₂O₃.Fe₂O₃ or C₄AF), 6CaO.Al₂O₃.2Fe₂O₃ or C₆AF₂) and6CaO.2Al₂O₃.Fe₂O₃ or C₆A₂F). Other minor constituents present in thecalcium aluminate cement include magnesia (MgO), titanic (TiO₂),sulfates and alkalis.

Calcium Sulfate

Various forms of calcium sulfate as shown below may be used in theinvention to provide sulfate ions for forming ettringite and othercalcium sulfo-aluminate hydrate compounds:

Dihydrate—CaSO₄.2H₂O (commonly known as gypsum or landplaster)

Hemihydrate—CaSO₄.½H₂O (commonly known as stucco, plaster of Paris orsimply plaster)

Anhydrite—CaSO₄ (also referred to as anhydrous calcium sulfate)

Landplaster is a relatively low purity gypsum and can be used foreconomic considerations, when greater compressive strength is notcritical. Higher purity grades of gypsum could also be used. Landplasteris made from quarried gypsum and ground to relatively small particlessuch that the specific surface area is greater than 2,000 cm²/gram andtypically about 4,000 to 6,000 cm²/gram as measured by the Blainesurface area method (ASTM C 204). The fine particles are readilydissolved and supply the gypsum needed to form ettringite. Syntheticgypsum obtained as a by-product from various manufacturing industriescan also be used as an acceptable form of calcium sulfate in the presentinvention. The other form of anhydrous calcium sulfate may also be usedin the present invention instead of the preferred hemihydrate or gypsum.

Alkali Metal Salts of Citric Acid

In the present invention, use of alkali metal salts of citric acid suchas sodium or potassium citrate, makes mixes with relatively goodfluidity and which do not stiffen too quickly, i.e., do not stiffenfaster than 5-10 minutes after mixing at temperatures above roomtemperature, while achieving good early age compressive strength.

The dosage of alkali metal salt of citric acid, e.g. potassium citrateor sodium citrates, is about 1.5 to 6.0 wt. %, preferably about 1.5 to4.0 wt. %, more preferably about 2.0 to 3.5 wt. % and most preferablyabout 3.5 wt % based on 100 parts of the cementitious reactivecomponents of the invention. Thus for example, for 100 pounds ofcementitious reactive powder, there may be about 1.5 to 4.0 total poundsof potassium and/or sodium citrates. The preferred alkali metal citratesare potassium citrates and sodium citrates and particularlytri-potassium citrate monohydrate and tri-sodium citrate monohydrate.

The use of the alkali metal salts of citric acid e.g., sodium orpotassium citrate, also provides for good fluidity and prevents theslurry from stiffening too quickly. Thus the use of retarders likecitric acid, tartaric acid, malic acid, acetic acid, boric acid, etc.can be avoided.

Air Entraining Agents and Superplasticizers

Air entraining agents are added to the cementitious slurry of theinvention to form air bubbles (foam) in situ. Air entraining agents aretypically surfactants used to purposely trap microscopic air bubbles inthe concrete. Alternatively, air entraining agents are employed toexternally produce foam which is introduced into the mixtures of thecompositions of the invention during the mixing operation to trap needleshape hydration products in the bubbles to produce a lightweightaggregate product. Typically to externally produce foam the airentraining agent (also known as a liquid foaming agent), air and waterare mixed to form foam in a suitable foam generating apparatus.

Alpha-olefin sulfonate is a type of anionic surfactant processed byalpha-olefin gas-phase sulfonation and continuous neutralization.Advantages of the use of this sulfonate include good compatibility, richand fine foam, easy biodegradation, low toxicity and low irritation toskin. WITCONATE™ AOS manufactured by Akzo Nobel was found to beeffective in laboratory testing to be highly effective in foaming flyash and sodium citrate binders to produce the aggregate product of thisinvention.

While the use of the AOS soap is preferred, other examples of airentraining/foaming agents that can be used include alkyl sulfonates,alkylbenzolfulfonates and alkyl ether sulfate oligomers among others.Details of the general formula for these foaming agents can be found inU.S. Pat. No. 5,643,510 incorporated herein by reference.

An air entraining agent (foaming agent) such as that conforming tostandards as set forth in ASTM C 260 “Standard Specification forAir-Entraining Admixtures for Concrete” (Aug. 1, 2006) can be employed.Such air entraining agents are well known to those skilled in the artand are described in the Kosmatka et al “Design and Control of ConcreteMixtures,” Fourteenth Edition, Portland Cement Association, specificallyChapter 8 entitled, “Air Entrained Concrete,” (cited in US PatentApplication Publication No. 2007/0079733 A1). Commercially available airentraining materials include vinsol wood resins, sulfonatedhydrocarbons, fatty and resinous acids, aliphatic substituted arylsulfonates, such as sulfonated lignin salts and numerous otherinterfacially active materials which normally take the form of anionicor nonionic surface active agents, sodium abietate, saturated orunsaturated fatty acids and salts thereof, tensides,alkyl-aryl-sulfonates, phenol ethoxylates, lignosulfonates, resin soaps,sodium hydroxystearate, lauryl sulfate, ABSs (alkylbenzenesulfonates),LASs (linear alkylbenzenesulfonates), alkanesulfonates, polyoxyethylenealkyl(phenyl)ethers, polyoxyethylene alkyl(phenyl)ether sulfate estersor salts thereof, polyoxyethylene alkyl(phenyl)ether phosphate esters orsalts thereof, proteinic materials, alkenylsulfosuccinates,alpha-olefinsulfonates, a sodium salt of alpha olefin sulphonate, orsodium lauryl sulphate or sulphonate and mixtures thereof.

Typically the air entraining (foaming) agent is present at about 0.01 to1.0 wt. % based upon the weight of the overall cementitious composition.

Although conventional superplasticizers can be used, there use isoptional. It has been found superplasticizers are not needed in thecurrent invention, since reduced water demand of the mixture is achievedwith the use of the alkali metal citrates of the invention.

Initial Slurry Temperature

In the present invention, slurry is formed to make the composition forsetting in situ to form aggregate of the present invention, which issubsequent recovered from the set composition by suitable recoveryprocess, such as by crushing or scraping of the set mixture. Forming theslurry under conditions which provide an initially high slurrytemperature was found to be important to achieve rapid hardening of thecementitious formulations used to make the aggregate particles. Theinitial slurry temperature should be at about room temperature to about41° C. Slurry temperatures in the range of 38° C. to 41° C. produceshort setting times, and are therefore preferred.

In general, within this range increasing the initial temperature of theslurry increases the rate of temperature rise as the reactions proceedand reduces the setting time. Thus, an initial slurry temperature of 95°F. (35° C.) is preferred over an initial slurry temperature of 90° F.(32.2° C.), a temperature of 100° F. (37.7° C.) is preferred over 95° F.(35° C.), a temperature of 115° F. (41.1° C.) is preferred over 100° F.(37.7° C.), a temperature of 110° F. (40.6° C.) is preferred over 115°F. (41.1° C.) and so on. It is believed the benefits of increasing theinitial slurry temperature decrease as the upper end of the broadtemperature range is approached.

As will be understood by those skilled in the art, achieving an initialslurry temperature may be accomplished by more than one method. Perhapsthe most convenient method is to heat one or more of the components ofthe slurry. In the examples, the present inventors supplied water heatedto a temperature such that, when added to the dry reactive powders andunreactive solids, the resulting slurry is at the desired temperature.Alternatively, if desired the solids could be provided at above ambienttemperatures. Using steam to provide heat to the slurry is anotherpossible method that could be adopted.

Although potentially slower, a slurry could be prepared at ambienttemperatures, and promptly (e.g., within about 10, 5, 2 or 1 minutes)heated to raise the temperature to about 90° F. or higher (or any of theother above-listed ranges), and still achieve benefits of the presentinvention.

Manufacturing Lightweight Aggregate of the Present Invention.

The aggregate of the present invention is formed by a method comprising,providing a lightweight cementitious mixture for making aggregateparticles, said providing comprising forming a mixture by mixing water,cementitious reactive powder comprising fly ash and a calcium sulfateselected from the group consisting of calcium sulfate hemihydrate,calcium sulfate dihydrate, and mixture thereof, an alkali metal salt ofcitric acid selected from the group consisting of sodium citrate,potassium citrate and mixtures thereof, and a foaming agent, wherein theaggregate particles comprising fly ash are formed in situ in themixture.

Once the slurry of the lightweight cementitious mixture is set in situ,the aggregate of the invention is recovered from the set mixture by useof a suitable recovery process, for example by crushing or scraping theset mixture to separate the aggregate particles from the set mixture.

One method used for entraining air in the fly ash binders is by addingthe foaming admixtures, i.e. alpha olefin sulfonate (AOS) soap, to thefly ash binders and generate the bubbles or air pockets in-situ whilethe fly ash binders were mixed for a predetermined amount of time.

The following foamed fly ash aggregate compositions described hereincontain 75-80.5% fly ash, 3 to 6% sodium (or potassium) citrate, 14 to20% water and 0.4 to 0.7% foaming agent as percent of the totalcomposition. The sodium citrate can be replaced with potassium citrateor a blend of both citrates can be used. The preferred foaming agent isstable soap made of long carbon chain (C₁₂-C₁₆) such as alpha olefinsulfonates and contain no ammonia to prevent the unwanted ammonia smellas the reaction is taking place during the mixing operation.

The successful production of the foamed fly ash binders for thelightweight aggregate of the invention optimizes the two followingparameters:

-   -   (a) Reaction rate of the fly ash binders, and    -   (b) Method for introducing foam to the fly ash/sodium citrate        binder.

Water, sodium citrate and foaming agent are first mixed homogeneously.These ingredients are added to the fly ash reactive powder. Foaming ofthis mixture starts immediately and is complete within 3 to 6 minutes.The mixture temperature rise starts soon after mixing, indicating anexothermic reaction as described in previous patent application by theauthor. Hardening at room temperature continues for the first 24 hr andfinal strength achieved within few days. The in-situ foam binders formedthis presents a unique combination of low weight associated withimproved compressive strengths.

The above components were combined using a weight ratio of water toreactive powder (fly ash) of 0.18/1 to 0.23. The microstructure of themixes was analyzed using a scanning electron microscope.

The wet density of the resulting mixture is within the range of about 40to 65 pounds per cubic foot, and more preferably about 46 to 51 poundsper cubic foot.

Manufacturing of Cementitious Products Using Lightweight Aggregate ofthe Invention

Lightweight aggregates such as blast furnace slag, volcanic tuff,pumice, expanded clay, shale, and perlite, hollow ceramic spheres,hollow plastic spheres, expanded plastic beads, and the like areconventionally used for the manufacture of lightweight concreteproducts.

Conventional expanded clay lightweight aggregates are also used inprecast concrete products such as cement boards which are made underrelatively high temperature conditions between 1200° to 2100° F. in arotary kiln. This process, known as “pyroprocessing” causes the claymaterial to expand into a synthetic lightweight aggregate having atypical bulk loose density of 40 to 65 pounds per cubic foot.

Precast concrete products such as cement boards are manufactured mostefficiently in a continuous process in which a reactive cementitiouspowder blend is blended with the lightweight aggregate of the presentinvention, with other necessary ingredients, followed by addition ofwater and other chemical additives just prior to placing the mixture ina mold or over a continuous casting and forming belt.

Due to the rapid setting characteristics of the cementitious mixture itshould be appreciated that the mixing of dry components of thecementitious reactive powder blend and present lightweight aggregatewith water usually will be done just prior to the casting operation. Asa consequence of the formation of the alkali alumino silicate hydratesand/or other hydrates of alumino silicates and/or calcium aluminosilicate compounds, the concrete product becomes rigid, ready forfurther processing and curing.

The products of the invention therefore also comprise cementitiouscompositions comprising the lightweight aggregate of an essentiallyhomogeneous mixture of spherical shaped particles of fly ash basedcementitious material and calcium sulfate of the invention and a binderof cementitious material, with the aggregate being distributed withinthe cementitious material.

EXAMPLE

The following example illustrates the influence of adding calciumsulfate dihydrate (landplaster) to the composition of the inventionincluding a mixture of class C fly ash and sodium citrate, on themicrostructure of the resulting binder which is used in the formation ofindividual lightweight aggregate particles of the invention.

Example 1

Compositions including, a mixture of class C fly ash, and calciumsulfate dihydrate (landplaster) or calcium sulfate hemihydrate (stucco)were used as the components of the reactive powder. The admixtures usedwere sodium citrate and a surfactant added as aqueous solutions. Theabove components were combined using a weight ratio of water to reactivepowder (fly ash plus calcium sulfate) of 0.2/1 to 0.287 depending on thewater demand of the calcium hemihydrate material. The microstructure ofthe mixes and the size of the lightweight aggregate were analyzed usingan optical microscope and SEM. Spherical aggregate particles are visibleintermixed in the concrete mix substantially immediately after mixing.

Granule samples were collected after the mixtures were harden bycarefully scraping these from few centimeters at the top surface of theharden mixtures. To study the specimens at relatively low magnification(FIGS. 1-6) optical images were captured through a computer attached toan Olympus SZX16 Research Stereo Microscope equipped with a DP71 CCDcamera. To study the microstructure of the aggregate particles at highmagnification (FIGS. 7-11) each sample was mounted on sample holders andgold coated under vacuum and analyzed using a JEOL Model JSM-840Ascanning electron microscope (SEM), made by JEOL USA, Inc., Peabody,Mass.

A solution containing 15 to 35% sodium citrate was made by stirringuntil the powdered sodium citrate dissolved. A foaming agent of 3-6%alpha olefin sulfonate soap (AOS) (Witconate AOS brand soap from AkzoNobel Company) was then added. Upon addition of the foaming agent, theviscosity of the sodium citrate solution increased significantly,indicating a synergistic interaction between the sodium citrate and thesoap. The sodium citrate/soap solution was then added to blends of classC fly ash and calcium sulfate dihydrate (gypsum/landplaster) or calciumsulfate hemihydrate (stucco) and mixed in a Hobart brand mixer at lowmedium speed for about 4-6 minutes at room temperature with no heating.

This example shows the effect of using class C fly ash blended withcalcium sulfate dihydrate (stucco) compared to class C fly ash blendedwith calcium sulfate hemihydrate (gypsum/landplaster) on the aggregateparticles obtained by the above-mentioned mixing procedure.

TABLE E shows the six compositions used in this example. In TABLE E, “g”means grams and W/S means the weight ratio of water to reactive powder(fly ash plus calcium sulfate). The wet density measured for these mixesranged from about 46 to 51 pcf (pounds per cubic foot).

TABLE E Class Wet C Fly Land- Extra Na AOS Density Mix Ash¹ g Stucco² gplaster³ g H₂O g H₂O g Citrate g soap g W/S Pcf. 1 800 200 0 150 79 457.5 0.229 48.2 2 700 300 0 150 107 45 7.5 0.257 47.4 3 650 350 0 150 13745 7.5 0.287 50.9 4 1000 0 0 150 50 45 7.5 0.2 46.9 5 700 0 300 150 5045 7.5 0.2 49.9 6 650 0 300 150 50 45 7.5 0.2 45.9 Campbell Class C flyash¹, CKS Stucco-Southard², Detroit Landplaster³

FIGS. 1-6 show the aggregate particles at 10× and 20× magnifications formixes 3, 5 and 6. The honeycomb surface of the aggregate particles isshown in the figures. The pictures in FIGS. 1 and 2 for mix 3 containinga blend of 65% class C fly ash and 35% stucco show relativelyhomogeneous particles around 1 mm or less in size. By contrast, thepictures in FIGS. 3 and 4 for mix 5 containing a blend of 70% class Cfly ash and 30% landplaster and FIGS. 5 and 6 for mix #6 containing ablend of 65% class C fly ash and 35% landplaster show a widedistribution of particles of from about 0.3 to 2 mm in size, with mostparticles being less than 1 mm in size. Thus relatively homogeneoussmaller particles are obtained when stucco is used in the blend with theclass C fly ash instead of landplaster.

The photographs in FIGS. 7-11 show SEM (scanning electron microscope)pictures with the characteristic microstructure of the cementitiousbinder and the aggregate particles formed using compositions shown inTable 1 for Mix 5 (FIGS. 7-9) and Mix 4 (FIGS. 10 a, 10 b, 11 a and 11b). FIGS. 7, 8 a, and 8 b show the microstructure inside the aggregateparticles (FIG. 8 a, b) is similar to the microstructure in the matrixoutside the aggregate. FIGS. 8 a and 8 b further show the detailedmicrostructure of the aggregate particles in FIG. 6 in which theaggregate particles comprising the fly ash and calcium sulfate formed insitu in the mixture also comprises a crystalline structure of hydrationproducts formed in situ from the reaction of the dissolution of some ofthe aluminates in the fly ash and calcium sulfate phase, which are alsoformed and interspersed within and between the aggregate particles tointerlock or bind the aggregate particles together to form the uniqueaggregate of this invention. FIG. 9 shows some of the gypsum particleswere trapped inside the aggregate particles. Thus, it appears theaggregate particles formed because some of the gypsum particles did nothave time to go into solution.

In the case where there was no gypsum or stucco present, as in the caseof Mix 4, there was a mostly glassy matrix without the abundance ofrelatively large crystals shown in FIGS. 7 and 8 a and 8 b for Mix 5,which contained gypsum. Only a small amount of crystalline phase isfound at the relatively higher 20× magnification in the photographs ofFIGS. 11 a and 11 b.

Although the preferred embodiments for implementing the presentinvention are described above, it will be understood by those skilled inthe art to which this disclosure is directed that modifications andadditions may be made to the present invention without departing fromits spirit and scope.

I claim:
 1. A method of making loose aggregate particles consisting ofthe steps of: forming a mixture by mixing ingredients consistingessentially of: water, cementitious reactive powder consistingessentially of fly ash and a calcium sulfate selected from the groupconsisting of calcium sulfate hemihydrate, calcium sulfate dihydrate,and mixtures thereof, and optional silica fume, an alkali metal salt ofcitric acid selected from the group consisting of sodium citrate,potassium citrate and mixtures thereof, a foaming agent, and optionallyat least one or more member selected from the group consisting ofsecondary aggregate other than said loose aggregate, superplasticizer,set accelerating agents other than said alkali metal salt of citricacid, set retarding agents, shrinkage control agents, thickening agents,coloring agents, and internal curing agents, and reacting the mixture insitu at a temperature of about 20° C. to 41° C. by mixing for about 3 to6 minutes to form aggregate particles comprising hydration products ofthe fly ash and calcium sulfate in the mixture and crystalline hydrationproducts of the fly ash and calcium sulfate which are also formed insitu and are interspersed within and between the aggregate particles tobind the aggregate particles, and separating the aggregate particles asloose aggregate particles from the mixture.
 2. The method of claim 1,wherein the wet density of the resulting mixture is about 40 to 65pounds per cubic foot, wherein the mixture has no hydraulic cement, andsaid ingredients consisting essentially of said water, said cementitiousreactive powder, said alkali metal salt of citric acid, and said foamingagent.
 3. The method of claim 1, wherein the fly ash in the reactivepowder comprises 88.5 to 100% class C fly ash, and wherein the mixturehas a set time of about 4 to 6 minutes.
 4. The method of claim 1,wherein the calcium sulfate is calcium sulfate dihydrate.
 5. The methodof claim 1, wherein the cementitious reactive powder is selected fromthe group consisting of class C fly ash and calcium sulfate hemihydrateor stucco, mixtures of class C and class F fly ash, calcium sulfatehemihydrate or stucco and/or Portland cement and mixtures of class F flyash with class C fly ash and/or Portland cement and calcium sulfatehemihydrate or stucco.
 6. The method of claim 1, wherein the wet densityof the resulting mixture is about 46 to 51 pounds per cubic foot;wherein the mixture has no hydraulic cement, wherein the ingredientsconsist of said water, said cementitious reactive powder, said alkalimetal salt, said foaming agent, and optional superplasticizer.
 7. Themethod of claim 1, wherein the alkali metal salt of citric acid is in anamount of about 1.5 to 6 wt. % based on the weight of the cementitiousreactive powder.
 8. The method of claim 1, wherein the cementitiousreactive powder comprises 60 to 95 wt. % fly ash and 5 to 40 wt. %calcium sulfate selected from the group consisting of calcium sulfatehemihydrate, calcium sulfate dihydrate, and mixtures thereof.
 9. Themethod of claim 1, further including setting the cementitious mixturecomprising the aggregate particles in situ and recovering the aggregateparticles from the set cementitious mixture by a suitable recoveryprocess to separate the aggregate particles from the set mixture. 10.The method of claim 1, wherein the cementitious reactive powder includesthe silica fume and has no Portland cement and no calcium aluminatecement, wherein the mixture has no other hydraulic cement, wherein theingredients consist of said water, said cementitious reactive powder,said alkali metal salt, and said foaming agent, and optionally a memberat least one member selected from the group consisting ofsuperplasticizers, set accelerating agents other than said alkali metalsalt of citric acid, set retarding agents, shrinkage control agents,thickening agents, coloring agents, and internal curing agents.
 11. Themethod of claim 1, wherein the cementitious reactive powder and waterare present in a weight ratio of about 0.200 to 0.287:1 part by weightwater to reactive powder.
 12. The method of claim 1, wherein thecementitious reactive powder and water are present in a weight ratio ofabout 0.22-0.287:1 part by weight water to reactive powder.
 13. Themethod of claim 1, for forming a lightweight cementitious aggregate:wherein the cementitious reactive powder consists of 60 to 95 wt. % flyash, and wherein the ratio of water to cementitious reactive powdersolids in the mixture is about 0.17 to 0.35:1.
 14. The method of claim13, wherein the cementitious reactive powder consists of fly ash whichis 88.5 to 100 wt. % Class C fly ash, calcium sulfate, optional silicafume and no Portland cement and no other hydraulic cement.
 15. Themethod of claim 13, wherein the amount of alkali metal salt of citricacid mixed into the mixture is about 1.5 to 6.0 wt. %, based upon theweight of cementitious powder.
 16. The method of claim 13, wherein thecementitious reactive powder is selected from the group consisting ofclass C fly ash and calcium sulfate hemihydrate or stucco, mixtures ofclass C and class F fly ash and calcium sulfate hemihydrate or stucco;and mixtures of class F fly ash with class C fly ash and/or Portlandcement and calcium sulfate hemihydrate stucco.
 17. The method of claim1, wherein the fly ash of the cementitious reactive powder consists of30-46 wt % class F fly ash and 54-70 wt % class C fly ash and themixture has no portland cement and no other hydraulic cement.
 18. Themethod of claim 1, wherein the cementitious reactive powder consists of46 to 60 wt. % Class F fly ash, 10 to 32 wt. % Class C fly ash, and10-29 wt % calcium sulfate hemihydrate or stucco and wherein the weightratio of water to the total weight of fly ash and calcium sulphate isless than about 0.33 to 0.37.
 19. The method of claim 1, wherein theaggregate particles consist of a homogeneous mixture of spherical shapedparticles of the reaction of the fly ash and calcium sulfate having adiameter of less than about 1 mm formed in situ and the crystallinehydration product of said fly ash and calcium sulfate, which is alsoformed in situ and interspersed in and between the particles and bindingsaid particles together in the aggregate.
 20. A method of making looseaggregate particles consisting of: forming a mixture by mixingingredients consisting of: water, cementitious reactive powderconsisting of fly ash, optional hydraulic cement, optional non-fly ashmineral additive, optional added lime, a calcium sulfate selected fromthe group consisting of calcium sulfate hemihydrate, calcium sulfatedihydrate, and mixtures thereof, and optional silica fume, an alkalimetal salt of citric acid selected from the group consisting of sodiumcitrate, potassium citrate and mixtures thereof, a foaming agent, andoptionally at least one or more member selected from the groupconsisting of secondary aggregate other than said loose aggregate,superplasticizer, set accelerating agents other than said alkali metalsalt of citric acid, set retarding agents, shrinkage control agents,thickening agents, coloring agents, and internal curing agents, andreacting the mixture in situ at a temperature of about 20° C. to 41° C.by mixing for about 3 to 6 minutes to form aggregate particlescomprising hydration products of the fly ash and calcium sulfate in themixture and crystalline hydration products of the fly ash and calciumsulfate which are also formed in situ and are interspersed within andbetween the aggregate particles to bind the aggregate particles, andseparating the aggregate particles as loose aggregate particles from themixture.